1. Field of the Invention
The present invention relates to can bodies having one or more axial beads or ribs formed in their side walls and also having step shoulder bottoms. More particularly, the present invention relates to a method for forming at least one rib and a step shoulder bottom of a can body in a single drawing operation from a redrawn cup which has a closed end with an inwardly tapering annular periphery including sufficient material to enable formation of the step shoulder bottom without having the at least one rib drawn into the step shoulder bottom.
2. Description of the Related Technology
Ribs or beads in the side walls of can bodies formed, for example, to enhance the appearance of a can and/or to add side wall strength for supporting axially applied loads, are known in the art.
Similarly known is the formation of can bodies using draw-redraw forming techniques wherein a first draw is performed to create a cup having a first diameter and height and a second draw or redraw is performed using a punch and redraw die. The cup is placed over an annular cup-holding member or redraw pad and the closed bottom of the cup is held by the redraw pad and a flat face portion of the redraw die. The redraw pad is moved synchronously with the redraw die. The relative movement of the punch and the redraw die extends the cup to form a deeper cup having a reduced second diameter and a reduced side wall thickness. During the redraw operation, the draw pad and the flat face portion of the redraw die act as a holding face which influences the plastic flow of the cup material as it is redrawn.
Cans used for packaging food and other products may have a bottom configuration that includes a step shoulder defined by an outwardly extending annular projection in the outer circumferential area of the bottom closely adjacent to the can side wall. The can bottoms may also have one or more concentric beads spaced radially inward from the step shoulder.
A problem that may be encountered when using draw-redraw forming techniques to form cans having step shoulders and axial ribs or beads in the side wall is that the axial ribs tend to be drawn into the step shoulder. For aesthetic and other reasons, drawing the axial ribs into the step shoulder is unacceptable. U.S. Pat. No. 6,374,657 discloses a manufacturing process for cans having a bottom with a step shoulder, referred to in the '657 patent as a bump-up bottom, and axial side wall ribs that ensures ample material in the bottom area of a can body preform so as to prevent the side wall ribs from being drawn into the step shoulder by providing a cup with a recessed bottom.
The '657 patent teaches two alternate two step processes. In the preferred process, the first step is a first drawing operation that creates a cup having a side wall that has at least one axially extending rib or bead formed therein and a bottom that is coextensive with the side wall and intersects the side wall at a rim. The first step is performed so that at least a portion of the bottom is recessed by a predetermined depth with respect to the rim. The second step is a second drawing operation performed on the cup to form a can body having a step shoulder or bump-up bottom wherein the predetermined depth of the recessed bottom of the cup is sufficient to prevent the at least one axially extending rib from being drawn into the bump-up bottom.
In an alternate process of the '657 patent, the first step is providing a cup having a sidewall and a bottom that is coextensive with the sidewall and intersects the sidewall at a rim, the first step being performed so that at least a portion of the bottom is recessed by a predetermined depth with respect to the rim. The second step is performing a forming operation on the cup to form a can body having at least one axial rib defined in its sidewall and having a bump-up bottom, and wherein the predetermined depth of the recessed bottom of the cup is sufficient so as to prevent the at least one axially extending rib from being drawn into the bump-up bottom during the second step.